The present invention is in the field of instrumentation and more particularly relates to apparatus for magnetically separating particles from a liquid medium.
The magnetic separation of solid material from a fluid medium has been accomplished in the prior art for processes where there was no concern for the integrity of the separated material. By way of example, processes are well known for separation of iron oxides from a mineral slurry. In practice, these separation processes lead to harsh physical interaction among the separated particles as well as between the separated particles and the separation matrix. Generally, there is no need in such fields of magnetic separation to be concerned about the intactness of the separated particles, although there is often concern with maintaining the integrity of the matrix.
In other applications of magnetic separation, there is a concern about integrity of separated particles. For example, there is the need to separate intact living, biological cells from a fluid carrier, so that those cells may be analyzed. As another example, a fragilely connected aggregate of particles may be considered as a "particle" for which separation from a carrier fluid is desired while maintaining the aggregate relationship. One known separation technique useful in these fields is high-gradient magnetic separation, HGMS.
In prior art HGMS systems, the collection of particles occurs on a matrix of magnetic wires, fibers, spheres or other high permeability members situated in a magnetic flux. Generally, such matrices are characterized by interstitial spaces through which the particles and carrier fluid may pass. As the particles pass through the matrix, each particle experiences a magnetic force toward the matrix elements proportional to EQU (.psi..sub.p -.psi..sub.f) V.sub.p H dH/dx,
where .psi..sub.p is the susceptibility of the particle, .psi..sub.f is the susceptibility of the carrier fluid, V.sub.p is the volume of the particle, H is the magnetic field intensity and x is a spatial dimension away from the matrix surface. In a paramagnetic mode of operation, where .psi..sub.p exceeds .psi..sub.f, that is where the particles are more "magnetic " than the carrier fluid, the particles are attracted to the elements of the matrix in the "strong field" regions at those elements. In a diamagnetic mode of operation, where .psi..sub.f exceeds .psi..sub.p, that is, where the carrier fluid is more "magnetic" than the particles, the particles are repelled from the strong field regions, but may be attracted to the weak or low field regions, at the matrix elements.
In a capture phase of operation, a fluid carrying the particles-to-be-separated is passed through the matrix at flow rates sufficiently low that magnetic attractive forces on the particles in the matrix exceed viscous and gravitational forces. As a consequence, those particles are held, or captured, against portions of the matrix while the carrier fluid exits the matrix. An elutriation phase may then be initiated to retrieve the captured particles from the matrix, for example, for subsequent analysis.
In HGMS systems where the magnetic flux is generated by an electromagnet, or by a permanent magnet whose flux is by some means removed from the matrix during the elutriation phase, particles can be released from the matrix following their collection from the particle-laden carrier by first interrupting drive current to the winding of the electromagnet, or removing the permanent magnet flux from the matrix. However, residual magnetism in the system may cause some particles to be held by the matrix. Then the velocity at which the elutriation fluid is driven through the matrix may be selectively increased to remove the non-released particles from the matrix.
In HGMS systems where the magnetic flux is generated by permanent magnets, and the matrix is maintained within the magnetic flux path at all times, that flux may continue to cause retention of the captured particles even upon the introduction of an elutriation fluid. The common method for elutriating the captured particles in this case is to appreciably increase fluid flow rates, so that the viscous drag forces exceed the magnetic retention forces; the captured particles are thus flushed off the matrix. This latter approach has been widely used with inorganic particles, but has been less successful when applied to separation of fragile particles such as intact living biological cells. Cellular debris observed in the flush effluent, particularly when old bloods are subjected to this method of cell elutriation, demonstrate that the method is too harsh for use with many clinical specimens.
It is an object of the present invention to provide an improved apparatus for magnetically removing particles from a fluid medium.
Another object is to provide an improved apparatus for magnetically capturing, and providing the intact removal therefrom of, fragile particles in a fluid medium.
Yet another object is to provide an improved apparatus for magnetically capturing, and providing the removal therefrom of, intact biological cells from a fluid medium.